net/phy: Add VSC8234 support

Vitesse VSC8234 is quad port 10/100/1000BASE-T PHY
with SGMII and SERDES MAC interfaces.

Signed-off-by: Andy Fleming <afleming@gmail.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Shruti Kanetkar <Shruti@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>